1. Field of the Invention
The present invention relates to the separation of gas and liquid from gas-liquid mixtures on a continuous basis, and relates more specifically to downhole gas separators used with sucker rod pumps in oil and gas wells.
2. Description of the Related Art
In oil and gas reservoirs, petroleum oil is frequently found in intimate association with natural gas, both in the form of free gas bubbles entrained in the oil and in the form of dissolved gas in the oil. Water is also commonly present in the reservoir fluids. Thus, well fluids commonly comprise both liquids and gas. In wells where pumping is necessary, the presence of this gas-liquid mixture materially affects the efficiency of pumping operations. In addition to the free gas in the mixture, the pressure decrease inherent at the suction of the pump inlet causes some of the dissolved gas to form more bubbles of free gas. The bubbles of free gas occupy part of the displacement of the pump, which results in reduced pumping efficiency. If the quantity of gas accumulates to a sufficient proportion, it will expand and contract to such a degree that the pump becomes gas locked, unable to cycle its flow control valves, and unable to pump any liquids at all.
A downhole reciprocating rod pump is the most common type of well pump being used today. Typically, the rod pump is run down inside the tubing string using a sucker rod string until it engages a seating nipple that is fixed to the tubing string, which then locates the inlet port of the rod pump at the depth of the seating nipple, and fixes the rod pump in position for pumping operation. The rod pump is then driven by a reciprocating surface unit through the string of sucker rods. The downhole pump pumps well liquids to the surface through the tubing string, while gas occupies an annulus between the tubing string and the well casing. The seating nipple and suction inlet of the pump are positioned below the liquid level in the well. In wells where bubbles of gas are present, it is known in the art to use a gas separator (“gas anchor”) to continuously separate the gas from the liquids before the liquid enters the inlet of the pump, the liquids being directed to the suction inlet of the pump and the gas being directed to the casing annulus. Thus, the gas separator is typically fluidly coupled to the suction inlet of the rod pump, and is therefore located below the rod pump itself. The efficiency of the separation of liquid and gas by the gas separator is a critical aspect of the gas separator design, and it should be noted that no gas separator is totally effective in this separation process.
Since prior art gas separators are located below the inlet of the downhole rod pump, the length of the rod pump and gas separator add together to establish the total depth below the well's natural liquid level that is required to properly submerge this equipment. Also, where the gas separator is below the rod pump, the liquid gas separation activity occurs below the pump as the liquids are drawn into the suction inlet of the pump by differential pressures. Thus, the length of the gas separator is related to the amount of differential pressure needed to draw the liquid and gas mixture through the gas separator and into the rod pump. This differential pressure is a negative pressure, which naturally draws some additional dissolved gasses out of solution. Any additional gases drawn out of solution at any point after the gas/liquid separation function of the gas separator has been completed, results in a direct reduction of pump efficiency since these gases must be compressed to at least the pump discharge pressure before any liquid is expelled from the pump.
In addition to the gas-liquid separation efficiency of the gas separator, it should be appreciated that the gas separator is typically located thousands of feet below the surface, so reliability is also critically important. It is further important for a gas separator design to facilitate its insertion and removal from the well bore casing using conventional oil field service systems and techniques. It is further important to address the practicalities of well field operations, including abusive handling practices, well fluid impurities, solids, abrasion, and unexpected failure of other well components. Given the high value of efficient oil and gas well production, the expense of operating and maintaining wells, and the cost of servicing well, it can readily be appreciated that there is a need in the art for cost effective, reliable, and efficient gas-liquid separators.